LECC 2006, ValenciaLECC 2006, Valencia Potential Upgrade of the CMS Tracker Analog Potential Upgrade of the CMS Tracker Analog

Readout Optical Links Using Bandwidth Efficient Readout Optical Links Using Bandwidth Efficient Digital ModulationDigital Modulation

Stefanos DrisImperial College London

CERN PH-MIC-OE

• A feasibility study for the development of a fast digital readout link for Super LHC (SLHC), based on the current CMS Tracker readout optical link components (first introduced at LECC 2005, Heidelberg).

• Why?A digital system based on the existing components that can deliver sufficient performance for SLHC operation could potentially be a cost-effective solution.

• How?The example of DSL: Advanced RF digital modulation is used to transfer data at 2-8Mbit/s over copper telephone lines (3-dB bandwidth of <30kHz).

• What is the data rate that we can achieve with bandwidth-efficient digital modulation over the current CMS Tracker optical link?

Objectives & MotivationObjectives & Motivation

IntroductionIntroduction

RF Modulation

OFDM

QAM Tests

Conclusion

The Current CMS Tracker Optical The Current CMS Tracker Optical LinksLinks

IntroductionIntroduction

RF Modulation

OFDM

QAM Tests

Analog Pulse Amplitude Modulation (PAM)Equivalent digital resolution = 8bitsSample rate = 40MSamples/sEquivalent digital data rate = 320Mbit/s

Conclusion

• The Shannon CapacityShannon determined that the capacity, C, of a noisy communication channel of bandwidth, B, is given by:

C=B*log2(1+SNR) [bits/s]

Digital CommunicationsDigital Communications

Introduction

RFRF ModulationModulation

OFDM

QAM Tests

3-dB bandwidth ~150MHz

(due to electronics)

SNR=48dB (over 100MHz)

Conclusion

• Quadrature Amplitude Modulation (QAM)

Passband Modulation: Data bits are modulated on top of a higher frequency sinusoidal carrier.

We transmit symbols which represent several bits each.

Each symbol corresponds to a combination of amplitude and phase. Compared to just varying the amplitude, more bits/symbol can be transmitted.

Quadrature Amplitude Quadrature Amplitude ModulationModulation

Introduction

RF ModulationRF Modulation

OFDM

QAM Tests

Am

plit

ud

e

3210Time

symbolperiod

Conclusion

• QAM is simply the simultaneous amplitude modulation of two sinusoidal carriers which are 90° to each other.

Quadrature Amplitude Quadrature Amplitude ModulationModulation

Introduction

RF ModulationRF Modulation

OFDM

QAM Tests

1101

11

01

Q=+3V

I=-1V

Cos(2πfct)

Sin(2πfct)

Q*Sin(2πfct)

I*Cos(2πfct)

I*Cos(2πfct) + Q*Sin(2πfct)

In-PhaseComponent

QuadratureComponent

Looks like Euler’s formula!ejω=cos(ω)+j sin(ω)

Conclusion

Quadrature Amplitude Quadrature Amplitude ModulationModulation

Introduction

RF ModulationRF Modulation

OFDM

QAM Tests

• We can represent QAM symbols in the complex plane since they each have a unique amplitude and a phase.

• It is used like an eye diagram in binary systems.

Conclusion

Quadrature Amplitude Quadrature Amplitude ModulationModulation

Introduction

RF ModulationRF Modulation

OFDM

QAM Tests

• Example:16-QAM received constellation diagram.log2(16)=4 bits/symbol

1000 symbols transmitted through a noisy channel

Qua

drat

ure

Am

plitu

de

In-Phase Amplitude

Conclusion

Quadrature Amplitude Quadrature Amplitude ModulationModulation

Introduction

RF ModulationRF Modulation

OFDM

QAM Tests

• The number of bits/symbol (4 bits/symbol) and the symbol rate (1MS/s) determine the total data rate (4Mbits/s), and channel bandwidth required (~4MHz).

Qua

drat

ure

Am

plitu

de

In-Phase Amplitude

Conclusion

Quadrature Amplitude Quadrature Amplitude ModulationModulation

Introduction

RF ModulationRF Modulation

OFDM

QAM Tests

• We can use more bits/symbol to increase the data rate in the same bandwidth, but at the cost of higher bit error rate (BER).

• 64-QAM: 6 bits/symbol, symbol rate=1MS/s, data rate=6Mbits/s

Qua

drat

ure

Am

plitu

de

In-Phase Amplitude

Conclusion

Multi-Carrier SystemsMulti-Carrier Systems

Introduction

RF Modulation

OFDMOFDM

QAM Tests

• A page out of the ADSL book: Orthogonal Frequency Division Multiplexing (OFDM)

Given the available bandwidth of our channel, we can:

a) Use a single QAM carrier with a high symbol rate occupying the entire frequency range.

b) We can split the channel into smaller slices. Each slice can have a low symbol rate QAM carrier.

In systems such as ADSL and Wi-Fi, multiple carriers are used.

-20

-15

-10

-5

0

Mag

nitu

de (

dB)

400350300250200150100500Frequency (MHz)

Conclusion

Multi-Carrier SystemsMulti-Carrier Systems

Introduction

RF Modulation

OFDMOFDM

QAM Tests

• A page out of the ADSL book: Orthogonal Frequency Division Multiplexing (OFDM)

Given the available bandwidth of our channel, we can:

a) Use a single QAM carrier with a high symbol rate occupying the entire frequency range.

b) We can split the channel into smaller slices. Each slice can have a low symbol rate QAM carrier.

In systems such as ADSL and Wi-Fi, multiple carriers are used.

-20

-15

-10

-5

0

Mag

nitu

de (

dB)

400350300250200150100500Frequency (MHz)

QAM SignalBandwidth=300MHz

QAM Modulated Carrier

Conclusion

Multi-Carrier SystemsMulti-Carrier Systems

Introduction

RF Modulation

OFDMOFDM

QAM Tests

• A page out of the ADSL book: Orthogonal Frequency Division Multiplexing (OFDM)

Given the available bandwidth of our channel, we can:

a) Use a single QAM carrier with a high symbol rate occupying the entire frequency range.

b) We can split the channel into smaller slices. Each slice has a low symbol rate QAM carrier.

In systems such as ADSL and Wi-Fi, multiple carriers are used.

-20

-15

-10

-5

0

Atte

nuat

ion

(dB

)

400350300250200150100500Frequency (MHz)

Conclusion

QAM Laboratory TestsQAM Laboratory Tests

Introduction

RF Modulation

OFDM

QAM TestsQAM Tests

• Objective

To test the feasibility of using QAM over a CMS Tracker readout optical link, and determine the data rate.

• We cannot test a high-speed multi-carrier system at this point (if we could…).

• We can, however, pass one QAM carrier at a time through the link: In each test, change the carrier frequency and analyze each carrier separately. Hence we are emulating an OFDM system by independently testing each of the carriers.

-20

-15

-10

-5

0

Cha

nnel

Ma

gnitu

de (

dB)

QAM SignalBandwidth~1-2MHz

Frequency (arbitrary scale)Fc

QAM CarrierCarrier Frequency = Fc

Signal Power = Ps

Conclusion

Test SetupTest Setup

Introduction

RF Modulation

OFDM

QAM TestsQAM Tests

• Modulator:

Agilent E4438C Vector Signal Generator.

Capable of QAM signal generation at symbol rates up to 50MSymbols/s, and 256-QAM (8bits/symbol). Maximum data rate = 50M*8 = 400 Mbits/s.

Generates random data patterns internally.

• Demodulator:

Agilent E4440A Spectrum Analyzer.

Capable of QAM demodulation (up to 256-QAM), with 10MHz analysis bandwidth.

Both instruments are made for telecom applications, and can work with carrier frequencies up to 27GHz.

Conclusion

Test SetupTest Setup

Introduction

RF Modulation

OFDM

QAM TestsQAM Tests

Conclusion

Example DataExample Data

Introduction

RF Modulation

OFDM

QAM TestsQAM Tests

• 32-QAM (5bits/symbol), Carrier Frequency=320MHz, Symbol Rate=1MS/s, Power=-30dBm

From the received constellations, we can estimate the SNR.

-0.8

-0.4

0.0

0.4

0.8

Qua

drat

ure

Am

plitu

de

-0.8 -0.4 0.0 0.4 0.8

In-Phase Amplitude

Conclusion

ResultsResults

Introduction

RF Modulation

OFDM

QAM TestsQAM Tests

• Signal to Noise Ratio as a function of carrier frequency and transmission power

We have essentially characterized the entire frequency range of the channel. This is all the information needed to calculate the achievable data rate using QAM over the Tracker optical links.

(See S. Dris et al., LECC 2005 for details of the calculation involved)

50

40

30

20

10

0

SN

R (

dB

)

10008006004002000Frequency (MHz)

-40

-35

-30

-25

-20 Inp

ut P

ow

er (dB

m)Conclusion

ResultsResults

Introduction

RF Modulation

OFDM

QAM TestsQAM Tests

Conclusion

• Achievable data rate for various target BERs

The total data rate achievable using QAM-OFDM also depends on the transmission power used which is implementation specific. The (most likely) achievable data rate is estimated at 3-4 Gbits/s.

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

Da

ta R

ate

(G

bits

/s)

013610Power Backoff (dB)

1.00.90.80.70.60.50.40.30.20.10.0

Normalized Transmit Power

10-3

10-5

10-7

10-9

• We have shown that data rates in the Gbit/s range are possible over the CMS Tracker optical links, using a QAM-based digital modulation scheme.

• Forward Error Correction (FEC) has not been considered in this study. We could expect a lower BER for the same data rate.

• This upgrade path would require creating a proprietary and novel system (i.e. forget about COTS components and ‘following’ industrial trends).

• More R&D and collaboration with industry and engineering departments will be necessary if we are to use this concept in future readout systems.

• What will be?

We have proved that the principle works, but the complexity and cost (as well as the projected power consumption) of hardware implementation needs to be investigated next.

We will then decide on whether this is worth pursuing further…

ConclusionConclusion

Introduction

RF Modulation

OFDM

QAM Tests

ConclusionConclusion